Poor error message with associated constrained trait type

[hellow554]

trait Bar {
    type Ok;
    type Sibling: Bar2<Ok=Self::Ok>;
}
trait Bar2 {
    type Ok;
}

struct Foo;
struct Foo2;

impl Bar for Foo {
    type Ok = ();
    type Sibling = Foo2;
}
impl Bar2 for Foo2 {
    type Ok = u32; // <- `u32` instead of `()`
}

(Playground)

Errors:

error[E0271]: type mismatch resolving `<Foo2 as Bar2>::Ok == ()`
  --> src/lib.rs:14:6
   |
14 | impl Bar for Foo {
   |      ^^^ expected u32, found ()
   |
   = note: expected type `u32`
              found type `()`

I don't like the error message, I would like to see something similar to this

error[E0271]: type mismatch resolving `<Foo2 as Bar2>::Ok == ()`
...

LN | type Ok = ();
   | ^^^^^^^^^^^^ associated type Ok defined here
LN | type Sibling: Bar2<Ok=Self::Ok>;
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type constrained here
   | ...
LN | type Ok = u32; // <- `u32` instead of `()`
   | ^^^^^^^^^^^^^ type violated here

[LukasKalbertodt]

A more simple example with the same "not so great" error message:

---

This code:

trait Bar {}

trait Foo {
    type Assoc: Bar;
}

impl Foo for () {
    type Assoc = bool;
}

Leads to (stable 1.32 and nightly 1.34):

error[E0277]: the trait bound `bool: Bar` is not satisfied
 --> src/lib.rs:7:6
  |
7 | impl Foo for () {
  |      ^^^ the trait `Bar` is not implemented for `bool`

The error is that bool does not implement Bar. I would expect that the error points to the line type Assoc = bool;, potentially with a good note. Example:

error[E0277]: the trait bound `bool: Bar` is not satisfied
 --> src/lib.rs:7:6
  |
8 |     type Assoc = bool;
  |     ^^^^^^^^^^^^^^^^^^ the trait `Bar` is not implemented for `bool`
  |
  = note: trait `Foo` requires the associated type `Assoc` to satisfy the bound `Bar`

[estebank]

With #65288, the case brought up in the last comment will look like this:

error[E0277]: the trait bound `bool: Bar` is not satisfied
 --> file8.rs:8:5
  |
4 |     type Assoc: Bar;
  |          ----- associated type defined here
...
7 | impl Foo for () {
  | --------------- in this `impl` item
8 |     type Assoc = bool;
  |     ^^^^^^^^^^^^^^^^^^ the trait `Bar` is not implemented for `bool`

and the original report like this:

error[E0271]: type mismatch resolving `<Foo2 as Bar2>::Ok == ()`
  --> file8.rs:13:5
   |
6  |     type Ok;
   |          -- associated type defined here
...
12 | impl Bar for Foo {
   | ---------------- in this `impl` item
13 |     type Ok = ();
   |     ^^^^^^^^^^^^^ expected u32, found ()
   |
   = note: expected type `u32`
              found type `()`

[LukasKalbertodt]

Thanks a bunch! :)

[hellow554]

On current stable (1.52.1) it looks like this:

error[E0277]: the trait bound `bool: Bar` is not satisfied
 --> src/lib.rs:8:5
  |
4 |     type Assoc: Bar;
  |                 --- required by this bound in `Foo::Assoc`
...
8 |     type Assoc = bool;
  |     ^^^^^^^^^^^^^^^^^^ the trait `Bar` is not implemented for `bool`

therefore I think this can be closed. Open it up again if you like to :)

[estebank]

@hellow554 because we can always improve things, #85799 :)

error[E0271]: type mismatch resolving `<Foo2 as Bar2>::Ok == ()`
  --> f23.rs:16:20
   |
5  |     type Sibling: Bar2<Ok=Self::Ok>;
   |                        ----------- required by this bound in `Bar::Sibling`
...
16 |     type Sibling = Foo2;
   |                    ^^^^ type mismatch resolving `<Foo2 as Bar2>::Ok == ()`
...
20 |     type Ok = u32; // <- `u32` instead of `()`
   |               --- expected this to be `()`

vs the current

error[E0271]: type mismatch resolving `<Foo2 as Bar2>::Ok == ()`
  --> src/lib.rs:16:5
   |
5  |     type Sibling: Bar2<Ok=Self::Ok>;
   |                        ----------- required by this bound in `Bar::Sibling`
...
16 |     type Sibling = Foo2;
   |     ^^^^^^^^^^^^^^^^^^^^ expected `()`, found `u32`